For certain ion implantation processes used in mass production of semiconductor manufacturing, such as cold, low-temperature, or sub-ambient temperature ion implantation processes, there is an increasing need to achieve high workpiece throughput that approaches the throughput typically associated with room-temperature ion implantation processing. Cold ion implantation processes typically cool workpieces to temperatures in the range of −70° C. to −100° C., wherein cooling of the workpiece may be achieved via cooling of the electrostatic clamp or chuck (ESC) on which the workpiece resides during ion implantation processing.
A fundamental problem with conventional technology is that in order to perform the cold ion implantation process, the workpiece is typically cooled for an initial period prior to the commencement of the ion implantation process. This cooling period, often referred to as a “dwell time”, is the period in which the workpiece is being allowed to reach a desired steady state temperature prior to starting the implantation process. The dwell time associated with pre-implant cooling is performed in series with other workpiece processing steps, and as such, increases the overall per-workpiece processing time.
There are two fundamentally different approaches in cooling the workpiece, which are mainly defined by the workpiece environment during the thermal transient associated with the pre-implant cooling. In a first approach, the workpiece can be placed at a so-called pre-implant chill station that is enclosed within a vacuum space associated with the ion implant process environment. At the pre-chill station, the workpiece is placed on a cooled wafer holder, whereby the workpiece is cooled to the desired process temperature. The workpiece is thereafter transferred to an electrostatic chuck (ESC) in the ion implant process environment to be scanned in front of an ion beam to undergo the ion implantation process.
In a second approach, the workpiece can be pre-cooled in the high-vacuum ion implant process environment, using the ESC or a chuck similar to the ESC used for the ion implantation process. Using this approach, the ESC can be configured to cool the workpiece prior to ion implantation (e.g., pre-implant cooling) and can also be configured to provide active in-situ cooling of the workpiece during the ion implantation process.
Conventionally, pre-implant cooling utilizing the ESC would employ a back side gas as a thermal media for cooling of the workpiece. In accordance with this in-situ cooling approach, the dwell time can typically be reduced as the workpiece can be brought to process temperature while in position for ion implantation, and possibly even during the commencement of the implantation process. However, the use of an in-vacuum ESC for parallel workpiece pre-implant cooling, in-situ cooling, and ion implantation can be very costly due to the cost associated with the technical modifications to the electrostatic chucking apparatus that are typically necessary to provide sufficient workpiece cooling while maintaining electrostatic forces sufficient to hold the workpiece in a fixed position while being scanned during the ion implant process.
Both of these conventional approaches suffer from the fact that when the workpiece is placed in a pre-implant vacuum environment for cooling, or is placed in the process environment, which is also a vacuum environment, heat transfer between the workpiece and ESC is limited by the thermal conductivity of the workpiece-to-cooling platen or chuck interface, which is determined by the back-side gap size, quality of contact between the workpiece and the chuck, and back side gas pressure.